Technical Field
The present disclosure relates to a three-dimensional measurement device configured to execute three-dimensional measurement by using a phase shift method.
Description of Related Art
In general, when electronic components are mounted on a printed circuit board, solder paste is printed on a predetermined electrode pattern provided on a printed circuit board. The electronic components are then temporarily fastened on the printed circuit board by means of the viscosity of the solder paste. The printed circuit board is subsequently introduced into a reflow furnace and is subjected to a predetermined reflow process to achieve soldering. Recently there has been a need to inspect the printing condition of solder paste in a stage prior to introduction into the reflow furnace. A three-dimensional measurement device may be used for this inspection.
Various contactless three-dimensional measurement devices using light have been proposed lately. Among them, three-dimensional measurement devices employing the phase shift method are known well.
In a three-dimensional measurement device using the phase shift method, a predetermined striped pattern is projected on a measurement object by a predetermined projection unit. The projection unit includes a light source configured to emit a predetermined light and a grid configured to convert the light from the light source into a striped pattern.
The grid is configured such that light transmitting portions of transmitting light and light shielding portions of shielding light are alternately arranged.
The three-dimensional measurement device then uses an imaging unit placed immediately above the measurement object to take an image of the striped pattern projected on the measurement object. The imaging unit used may be, for example, a CCD camera comprised of a lens, an imaging element and the like.
In the configuration described above, the intensity (luminance) I of light in each pixel on image data taken by a camera is given by Expression (U1) given below:I=f·sin φ+e  (U1)where f denotes a gain, e denotes an offset and φ denotes a phase of a striped pattern.
A related art is configured to intermittently move a grid, shift the phase of a striped pattern, for example, by 90 degrees each to four different phases (φ+0, φ+90°, φ+180° and φ+270°), and take an image of the striped pattern every time the grid is at stop as shown in FIG. 28. This provides image data having intensity distributions I0, I1, I2 and I3 taken under the respective striped patterns having different phases and then determines the phase φ according to Expression (U2) given below:φ=tan−1[(I1−I3)/(I2−I0)]  (U2)
A height (Z) at each coordinates (X,Y) on a measurement object can be determined by using this phase φ, based on the principle of triangulation.
In general, a three-dimensional measurement device using the phase shift method projects a striped pattern having a light intensity distribution in a sinusoidal waveform, with a view to increasing the measurement accuracy. It is, however, very difficult to project a striped pattern having a light intensity distribution in a sinusoidal waveform of a high accuracy.
A recently proposed technique projects a striped pattern having a light intensity distribution in a rectangular waveform that is obtained by conversion via a grid, on a measurement object with defocusing, so as to project this striped pattern as a striped pattern having a light intensity distribution in a sinusoidal waveform (as described in, for example, Patent Literature 1).